1. Field of the Invention
The present invention relates to a rotating apparatus and a method for manufacturing the rotating apparatus and, more particularly, to a technology for improving the shock resistance of such a rotating apparatus.
2. Description of the Related Art
The demand for rotating apparatuses represented by hard disk drives (hereinafter referred to as “disk drive device”, or “HDD” also) today is to meet the double requirement of further downsizing and reliability of withstanding high impact accelerations.
Many of the disk drive devices are equipped with a bearing unit that rotatably supports a hub having recording disks fit thereon. The bearing unit has a radial dynamic pressure groove on at least a shaft, which serves as a rotating shaft, and/or a shaft housing member, which houses the shaft, and a lubricant filled in a gap between the shaft and the shaft housing member. As an example of a structure of such a bearing unit, there is a bearing unit structure called a shaft fixing type, which is disclosed in Japanese Unexamined Patent Application Publication No. 2000-197305. This bearing unit is of such structure that the shaft housing member is joined to the interior of the hub, and the shaft is fixed to a bearing hole in a base plate.
In the disk drive device as described above, recording disks of magnetic recording type with recording tracks formed thereon are supported by the bearing unit joined to the base plate, and the recording disks are rotated at high speed. In pursuing the downsizing of such rotating apparatus as the disk drive device, and especially the thinning of the disk drive device in the direction of the axis (shaft) of rotation, due consideration must therefore be given to the fact that the shock resistance of the disk drive device depends very much on the performance of a joint between the bearing unit and the base plate.
It is to be noted also that the disk drive device related to a fixed-shaft type such as that mentioned above, when made thinner, generally results in the thinning of the joined portion of the shaft and base plate. However, the joined portion of the shaft and the base plate, if made thinner, may become deformed and thus increase the tilt of the hub when impact acceleration acts on the disk drive device. The increased tilt of the hub raises the frequency of read/write errors of the disk drive device.
On the other hand, as market surveys and the like have proven, the fixed-shaft type disk drive device is viable on the market provided the disk drive device thereof has a shock resistance performance capable of withstanding test impact acceleration of 400 G. For example, when impact acceleration of 400 G is outwardly applied on a disk drive device having a hub fit with four recording disks whose total mass is 150 g, it is assumed that a load of about 600 N works on the joined portion of the shaft and the base plate. In designing disk drive device, it is customary that a safety margin of about 30 percent be incorporated. That is, it is desirable that the structure is designed on the assumption that a load of 800 N works on the joined portion of the shaft and the base plate.
For the disk drive device having recording disks whose total mass is about 150 grams, therefore, the desired structure has a binding force capable of retaining the functions of the disk drive device even under a test load of 800 N to the joined portion of the shaft and the base plate. In other words, the structure must be such that the binding force is strong enough to keep the hub under a predetermined maximum tilt value even when a test load of 800 N is applied to the joined portion of the shaft and the base plate. Thus, there is reduced inclination of the recording disks and, therefore, reduced frequency of read/write errors.
Increasing the axial dimension of the joined portion of the shaft and the base plate may be conceivable as a way for raising the binding force of the joined portion thereof. Such a structure, however, may increase the thickness of the disk drive device, thus making it hard to achieve thinning, which is one of the purposes of the present invention. Another conceivable way may be the use of a thicker shaft. In this case, however, the larger diameter of the shaft may also increase the diameter of the hub and may thus increase the weight thereof. As a result, the load during rotation will increase, consequently requiring a larger drive current. Also, the overall size of the bearing unit may increase, and the inside diameter of the laminated core may be correspondingly larger. And for the same overall size of the laminated core, there is a narrower space for coil winding and fewer turns of the coil, with a result of the drive current possibly increasing. This will go against a separate requirement for electric power saving.